At present, refrigeration systems, including those for cryogenic applications, are almost entirely based on compression and expansion cycles of a gas. Generally, the efficiency of practical gas cycle refrigerators is only a fraction of the ideal Carnot efficiency, and the efficiency generally decreases with a decrease in the size of the refrigerator. The efficiency of gas cycle refrigerators is particularly low at cryogenic temperatures, e.g., in the 2 K. to 20 K. range. Reliability can also be a problem with large refrigeration systems operating to about 2 K.
It has long been known that certain magnetic materials exhibit the magnetocaloric effect: they increase in temperature when placed in a magnetic field and decrease in temperature when removed from the field. Application of a magnetic field to such solid magnetic materials is analogous to compressing a gas (producing an increase in temperature) and removing the field is analogous to expanding a gas (producing a decrease in temperature). Thus, it has been recognized that a thermodynamic refrigeration cycle can be achieved using a magnetic material as the working material in a manner analogous to the refrigeration cycles of a gas. Refrigerators utilizing the magnetocaloric effect require several essential components. A magnetic material that exhibits a magnetocaloric effect suited to the intended operating temperature range is the refrigerator's working material. Magnets of sufficient field strength to produce the necessary field changes at the working material are required. Means for effecting the necessary cyclic changes in magnetic field at the working material must be included. Switches enabling heat transfer and heat transfer modes to transfer heat to and from the working material at requisite locations within the refrigerator are necessary. A thermal source from which heat is extracted is necessary, as is a sink to which heat is rejected. Finally, a structure with appropriate thermal, magnetic, and physical properties to support the essential elements of the refrigerator with minimum negative performance impact must be included. Examples of relatively recent designs proposed for magnetic refrigerators are shown in U.S. Pat. Nos. 4,033,734, 4,069,028, 4,107,935, 4,332,135, 4,392,356, 4,408,463, 4,441,325, 4,457,135, 4,459,811, 4,464,903, 4,507,927, 4,507,928, and 4,702,090.
A common configuration of magnetic refrigerators is to have the magnetic material attached to or shaped like a wheel, ring, piston, or the like, and cyclically move the magnetic material through a stationary magnetic field of alternating strength to alternately magnetize and demagnetize the magnetic material. For high efficiency, excellent heat transfer between the magnetic material and the heat transfer fluid is required. This usually means the magnetic material must be in a geometry which has large surface area. The magnetic material is often closely toleranced to move through and connect with the thermal source and the heat sink for the necessary heat transfer toward and away from the magnetic refrigerator. The magnetic material is sometimes shaped to include fins for increased heat transfer performance, thus making the required high tolerances even more difficult to achieve. Magnetic refrigerators typically have a number of moving parts, require moving parts in the area of heat transfer fluid, and the magnetic material requires close tolerances. Though these problems are not insurmountable, they may require fastidious attention in the design and fabrication of the magnetic refrigerator.
An alternate approach that has been suggested is to employ a fixed magnetic regenerative bed in which the magnetization and demagnetization is accomplished by using charge/discharge magnets. However, such charge/discharge magnets have been found to compromise the overall efficiency of the magnetic refrigerator.